Replace String from particular position

I want to have a string function that replaces the substring with anotherset of characters - but the condition is that i want a particularsubstring to be replaced if there are multiple same substring exits.e.g.

Original_String :='I am an Indian'

Character/substring to replace 'a'

Required_String := 'I am an IndiaXn'

can you see - there are multiple 'a's into Original_String, howeverI just want to replace the last a with X.

I tried native replace_string function but that replaces all 'a's - instead I want particular one...